1. Field of the Invention
The present invention relates to a motor control apparatus that controls a rotational position of an electric motor, which changes an operational position of a control subject.
2. Description of Related Art
Lately, even in the automobile industry, many mechanical drive systems have been replaced with electrical drive systems, each of which uses an electric motor as a drive source thereof to satisfy a demand for improving installation space saving of the system, a demand for improving an easiness of assembling of the system, a demand for improving a controllability of the system and/or the like. For example, a range change mechanism of an automatic transmission of a vehicle has been changed to use an electric motor as a drive source thereof (see, for example, Japanese Unexamined Patent Publication No. 2002-323127). In this case, an output shaft is connected to a rotatable shaft of the electric motor through a speed reducing mechanism, and the range change mechanism is driven by this output shaft to change the range of the automatic transmission. In this case, an encoder, which senses a rotational angle, is provided to the motor. At the time of changing the range, the motor is rotated to a target position (a target count value), which corresponds to a target range based on a count value of output pulses of the encoder
The rotational amount (a rotational angle) of the motor is converted into the rotational amount of the control subject (the operational amount of the range change mechanism) through a rotation transmission system, such as the speed reducing mechanism. Here, it should be noted that a play (looseness) exists between adjacent components of the rotation transmission system. For example, a play (backlash) exists between adjacent gears of the speed reducing mechanism. Furthermore, in a case where a connecting portion, which has a non-circular cross section (e.g., a polygonal cross section, a D-cut cross section) and is provided to a distal end portion of the rotatable shaft of the speed reducing mechanism, is fitted into an engaging hole of a connecting shaft of the control subject, a clearance is required to ease the fitting work for fitting the connecting portion of the rotatable shaft into the engaging hole of the connecting shaft of the control subject. Due to the presence of the play (looseness) in the rotation transmission system, which converts the rotational amount of the motor into the operational amount of the control subject, even when the rotational angle of the motor is correctly controlled based on a sensed value of a rotational angle sensor, an error, the amount of which corresponds to the play (looseness) of the rotation transmission system, is created in the rotational angle of the connecting shaft of the control subject (the operational amount of the range change mechanism). Thereby, the operational amount of the range change mechanism cannot be accurately controlled.
In view of the above disadvantage, as disclosed in Japanese Unexamined Patent Publication No. 2004-23932 (corresponding to U.S. Pat. No. 7,221,116), the amount of play in the rotation transmission system is learned by executing an abutment control operation, in which the motor is rotated until abutment to a delimiting position (a wall) of a movable range of the range change mechanism is made. Then, a target position is set (corrected) in view of a learning value of the delimiting position, which is learned through the abutment control operation.
Furthermore, as in the case of Japanese Unexamined Patent Publication No. 2004-23932 (corresponding to U.S. Pat. No. 7,221,116) where the motor is rotated slightly beyond the delimiting position upon the abutment of the motor to the delimiting position, an exceeding rotational angle of the motor, which exceeds the delimiting position, is increased as the torque of the motor is increased. In general, the torque of the motor changes depending on the electric power source voltage of the motor. Therefore, in the case of Japanese Unexamined Patent Publication No. 2004-23932 (corresponding to U.S. Pat. No. 7,221,116), a climb correction amount is provided to consider the exceeding rotational angle of the motor, which exceeds the delimiting position of the motor, upon the abutment of the motor to the delimiting position at the time of executing the abutment control operation. The climb correction amount is set based on the electric power source voltage of the motor, and the target position is corrected by the climb correction amount.
The torque of the motor is changed depending on the electric power source voltage of the motor and is also changed depending on the temperature of the coil (hereinafter, also referred to as the coil temperature) of the motor. When the coil temperature of the motor is increased, a resistance value of the coil is increased. Therefore, even in the case where the same power source voltage of the motor is applied, when the coil temperature of the motor is increased, the torque of the motor is reduced. Thus, the climb correction amount is changed depending on the temperature of the motor, and the learning accuracy of the amount of play is reduced to cause a reduction in the position change accuracy of the control subject.